It is well known that during the process of acid treating a subterranean formation, contamination of the treating acid by dissolved iron or by iron compounds is inevitable. An article appearing in Sept. 3, 1984 issue of the Oil & Gas Journal (Page 87-88) indicates the level of iron typical of various sources. It is concluded that the principal source is the mill scale and rust on the steel tubulars that form the conduit from the surface to the formation. It concludes that in the absence of an acid prewash, levels of 9,000 to 100,000 mg/L iron can occur. When an acid washing treatment is carried out prior to the formation treatment, 1,000 to 2,000 mg/L of dissolved iron should be the level of iron entering the formation. A more recent paper (#13) presented Oct. 6, 1987 to the Petroleum Society of Canadian Institute of Mining in Regina, Saskatchewan indicates that a small volume acid wash carried out just prior to the acid treatment can still result in iron levels of 500 to 7,000 mg/L in the acid contacting the formation. It should also be noted that low reservoir pressure, acid disposal problems, or the presence of a permanent packer around the tubing may make it impossible to conduct an acid wash prior to the acid treatment. It is therefore important to take precautions to minimize the affects of ferric and ferrous iron contamination in the treatment acid.
U.S. Pat. No. 1,137,290 reviews a partial list of iron problems and their control. From the background provided, the most widely recognized problem is the precipitation of ferric hydroxide as the acid spends on acid soluble scale or formation components. Ferrous hydroxide being much more soluble does not normally constitute a problem. The use of sequestering agents such as citric acid, ethylene diamine tetra-acetic acid (EDTA) or nitrilo triacetic acid (NTA) were described as the most common agents for controlling the precipitation, although their effectiveness at temperatures above 125.degree.-150.degree. F. is poor. Another problem referred to is the precipitation of insoluble iron asphaltene compounds. U.S. Pat. No. 4,096,914 discloses the reaction of the ferric iron in hydrochloric acid strengths of 0.1 to 2.0 molar (0.4 to 7.0 wt %) with toluene solutions of Ventura asphaltic crude oil. Sequestering of the ferric acid in acid solution using salicylic acid was proposed as a method of control. The highest level of ferric iron tested was 3,400 mg/L. The use of monosodium phosphinite was disclosed and shown to reduce the sludge from 250 mg down to 18 mg. Although this is impressive, the solutions were aged for 20 hours at 70.degree. C. prior to testing. This is not representative of most acid treatment conditions.
In U.S. Pat. No. 4,137,972 the use of sulfosalicylic acid is proposed as an improvement over salicylic acid proposed although only comparative complex ion formation data was given and only on solutions with pH values 3-10.
Referring again to U.S. Pat. No. 1,137,290, it is claimed that ascorbic acid, erythorbic acid and/or their salts have utility in minimizing both the ferric hydroxide problem and the iron asphaltene problem. No data however was given to show their effectiveness in preventing iron asphaltene precipitates.
A more exhaustive description of the problems caused by iron contamination in acidizing is contained in a paper (#85-36-38) presented to the Petroleum Society of Canadian Institute of Mining in Edmonton, June 1985, and also slightly modified in a paper (#TT-107) presented to the Third Petroleum Congress of Brazil, October 1986, in Rio de Janeiro. In those papers data is presented to show that, in addition to ferric hydroxide precipitation on spending and iron asphaltene precipitation (iron induced sludging), iron contamination (notably ferric) can also adversely react with other acid additives. The reaction appears to involve the formation of complexes between the iron and oxygen-containing organic compounds such as ethoxylated surfactants. The result of this reaction is that a normally dispersible blend of acid additives can, when iron contamination is present, separate from solution as glue like droplets containing a high dissolved iron content. Both the iron asphaltene precipitation and the iron/surfactant reactions were shown to be much more prevalent in 28% acid compared to 15% acid and much more pronounced for ferric iron compared to ferrous iron. In the absence of a totally effective chemical solution to the iron problems, several modifications to the field execution of acid treatments were recommended to minimize the level of iron contamination.
Two additional problems were reported in the April 1985 issue of The Journal of Petroleum Technology pages 691 to 695. In the case of sour wells it is apparent that any ferric iron present can oxidize sulfides to insoluble elemental sulfur deposits. Also, ferrous iron can form insoluble ferrous sulfides as the acid spends. This can be quite plugging and the use of chelating agents is required to minimize this latter effect. An effective reducing system is recommended to minimize all other iron problems.
The most recent art noted is that of U.S. Pat. No. 4,679,631, wherein dihydroxy maleic acid or its salts combined with gluconodeltalactone or boric acid or boric salts is found to be a reducing agent for ferric iron in fracturing fluids. The same composition is described as a complexing agent for ferric iron in acidizing fluids. No reference to the effectiveness of the composition in preventing the iron asphaltene precipitation in acid was made but it is logical to expect it to be negligible.
It is apparent then, that from current art, the best known methods of preventing ferric iron problems including iron asphaltene precipitation is through the use of erythorbates (ascorbic acid, erythorbic acid and their salts), or alternatively the use of sulfosalicylic acid.
Numerous tests by the inventors with Canadian crude have shown that while erythorbates are quite effective in preventing sludge in 10% hydrochloric acid and low concentrations of ferric iron their effectiveness drops off rapidly as the acid strength increases to 15%. As a result, excessive amounts of erythorbates must be used to control iron asphaltene sludge in 15% hydrochloric acid. In addition, erythorbates are unstable in hydrochloric acid and degrade fairly rapidly to tarry solids. The rate of degradation increases dramatically as the temperature of the acid is raised. This is a particularly serious problem when a staged acid treatment is being carried out. In this case, packers must be set and released across sections of the zone to be stimulated and generally small volumes of acid are injected during each stage. The acid in those cases can remain at reservoir temperature for one or more hours. During this time degradation of the erythorbates would most certainly occur, thereby limiting their use in deep well acidizing.
Sulfosalicylic acid on the other hand has been found ineffective in preventing iron asphaltene sludge in 15% hydrochloric acid with the Canadian oils tested. It is desirable in most instances to use 15% hydrochloric acid or higher to provide for increased penetration of live acid into the formation while minimizing the amount of spent acid that has to be produced back (unloaded) following the treatment.
There is therefore a need for more acid stable and effective agents for controlling the iron induced sludging of oils during acidizing.